This invention relates to crystals for radio-frequency devices, and in particular, to a dual crystal package for crystal oscillators that has two crystals that resonate at different frequencies.
Oscillators use a piezoelectric material such as a quartz crystal and electronic circuitry to provide a reliable and stable oscillator output signal. The crystal is mounted in a package. Oscillators and crystals are commonly found in portable radio frequency (RF) communication equipment, such as cellular telephones, pagers and wireless modems. In portable communications equipment, it is necessary to minimize the size of the crystal package as well as to minimize the power consumed by the device because of the use of battery power. One such application is in local communication device modules such as bluetooth that are designed into cell phones and laptop computers.
The amount of power consumed by a crystal resonator is proportional to the frequency of the oscillator. One way to reduce the power consumption in electronic equipment is to place the equipment in a sleep mode during periods of non-use. A further power reduction technique is to shift from using a high frequency oscillator during normal operation to a low frequency oscillator during the sleep mode. This has been previously done using two separate crystals and circuits mounted on a circuit board. Unfortunately, this technique uses more circuit board space. For example, a crystal resonator operating at 32 Megahertz will draw 75 milli-watts, while a crystal resonator operating at 32 Kilohertz will draw 0.075 milli-watts. The 32 MHz crystal consumes 1000 times more power than the 32 KHz crystal.
The height, width and depth of the package are dependent upon the size of the crystal, any circuitry and the package dimensions. In portable electronic devices, it is necessary to minimize the size of the device. The demand for smaller cellular telephones and other electronic products requires even smaller physical dimensions.
U.S. Pat. Nos. 6,229,249, 6,229,404 and 5,438,219 show a prior art crystal oscillator that has a crystal and electronic circuitry attached to a substrate. These crystal packages only contain a single crystal.
Therefore, a current unmet need exists for a crystal package that has a small size, that can hold two or more crystals, that can be easily mass produced at low cost and that maintains a stable frequency of operation over the life of the oscillator.
This invention overcomes problems of the prior art by providing a crystal package that contains two or more crystals that operate at differing frequencies. An embodiment of this invention is a crystal package that has a substrate having a top surface, a bottom surface, and side surfaces. A cavity is located in the top surface. A first step is located in the cavity. A second step is located in the cavity. A first crystal is mounted to the first step and extends over the cavity. A second crystal is mounted to the second step and extends over the first crystal.
There are other advantages and features of this invention which will be more readily apparent from the following detailed description of the preferred embodiment of the invention, the drawings, and the appended claims.